def CreateInterval(start,stop):
    interval=dates[(dates['Date']>=start)&(dates['Date']<=stop)]
    plt.bar(interval['Date'],interval['Trips'])
    plt.xlabel('Date')
    plt.ylabel('Number Trips')
    plt.title('Number of trips during the interval',start,stop)
    plt.show()

CreateInterval('2012-03-13','2012-04-13')